Process for the recovery of cobalt and tungstic acid and/or its derivatives from aqueous solutions

ABSTRACT

This invention relates to a process for the recovery of cobalt ions and tungstic acid and/or its derivatives from aqueous solutions, such as in particular the spent catalytic waters deriving from processes for the oxidative cleavage of vegetable oils. In particular this invention relates to a process for the recovery of cobalt ions and tungstic acid and/or its derivatives which provides for the use of cation-exchange resins.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. Application No. 15/542,852,filed on Jan. 20, 2016, which is the National Phase filing under 35U.S.C. § 371 of PCT/EP2016/051067 filed on Jan. 20, 2016; whichapplication in turn claims priority to Application No. MI2015A000060filed in Italy on Jan. 22, 2015 under 35 U.S.C. § 119. The entirecontents are hereby incorporated by reference.

DESCRIPTION

This invention relates to a process for recovering cobalt ions andtungstic acid and/or its derivatives from aqueous solutions, such as inparticular the spent catalytic waters deriving from processes for theoxidative cleavage of vegetable oils. In particular this inventionrelates to a process for the recovery of cobalt ions and tungstic acidand/or its derivatives, which provides for the use of cation-exchangeresins.

Vegetable oils are now an important raw material for the chemicalindustry on account of the ever more pressing need to identify rawmaterials of renewable origin which are alternatives to conventionalpetroleum sources.

For example WO 2008/138892 and WO 2011/80296 describe processes foroxidative cleavage, which, starting from vegetable oils containingmonounsaturated fatty acid triglycerides, make it possible to produceintermediates for the preparation of polyesters, such as for example thesaturated dicarboxylic acids, azelaic acid or brassylic acid. Typicallythe said processes provide for at least two immediately consecutivereaction stages: a first stage of hydroxylation of the double bondpresent in the monounsaturated fatty acid triglycerides to yield avicinal diol and a subsequent second stage of oxidation of the twohydroxyl groups of the vicinal diol into carboxylic groups, obtaining amixture comprising saturated monocarboxylic acids and saturatedcarboxylic acid triglycerides having more than one acid group.

These processes preferentially make use of different catalysts toincrease the reaction rate and in general improve the yield andselectivity of the two reactions involved. For example catalysts such astungstic acid and/or its derivatives, such as for example,phosphotungstic acid, are preferably used for the hydroxylation stage,while catalysts based on cobalt salts, such as for example cobaltacetate, cobalt chloride, cobalt sulfate, cobalt nitrate or cobaltbromide, are used for the oxidation stage. At the end of thehydroxylation stage, the catalyst is typically not separated from thereaction medium, and therefore mixes with the catalyst from theoxidation stage. Typically, at the end of the oxidative cleavage processthe spent catalytic waters which are separated from the reactionproducts contain cobalt ions and tungstic acid and/or their derivatives,such as for example phosphotungstic acid, pertungstic acid andpolytungstates, which may also be substituted with cobalt ions.

From the point of view of industrial production, the use of twodifferent catalysts in two immediately consecutive stages ofhydroxylation and oxidation therefore gives rise to the need to developtreatments to separate and recover the catalysts with a view to reusingthem in the process so as to help reduce the costs of disposing of thecatalytic waters, as well as the environmental impact of the processesthemselves.

Starting from this technical problem, it has now been discovered that itis possible to effectively recover both cobalt ions and tungstic acidsand/or their derivatives from an aqueous solution through a processcomprising the following stages:

-   a) removing the cobalt ions by placing the said aqueous solution in    contact with a cation-exchange resin;-   b) separating the said aqueous solution from the cation-exchange    resin;-   c) concentrating the said aqueous solution obtained from stage b,    obtaining a concentrated aqueous solution containing tungstic acid    and/or its derivatives;-   d) eluting cobalt ions from the cation-exchange resin in stage b,    using an acid aqueous solution.

The process according to this invention is particularly suitable forrecovering and effectively and economically separating cobalt ions andtungstic acid and/or its derivatives, such as in particularphosphotungstic acid, pertungstic acid and polytungstates, even whensubstituted with cobalt ions, from spent catalytic waters such as forexample those originating from the processes of oxidative cleavage ofvegetable oils such as those described in WO 2008/138892 and WO2011/80296. The process can also be used for the recovery and separationof cobalt ions and tungstic acid and/or its derivatives which are alsopresent in aqueous solutions of different origin. For simplicity ofdescription reference will always be made to the spent catalytic watersin the rest of the description, this term also being intended to includeany other aqueous solution having similar compositional characteristicswithout any distinction as to origin.

In stage a) of the process according to this invention the spentcatalytic waters from which it is intended to recover the cobalt ionsand tungstic acid and/or its derivatives are placed in contact with acation-exchange resin which is capable of adsorbing the said cobaltions. This stage may be performed in any equipment suitable for thepurpose known to those skilled in the art, such as for example vessels,stirred reactors and tanks, mixers and ion-exchange columns. In order tomaximise intimate contact between the cation-exchange resin and thewaters containing cobalt ions and tungstic acid and/or its derivatives,said stage a) is advantageously performed in one or more ion-exchangecolumns, which may also be arranged in ranks or in sequence, dependingupon the desired configuration for the process. For example, stage a) ofthe process according to this invention may be carried out using asingle ion-exchange column or two or more ion-exchange columns placed inseries. Depending upon the initial concentration of the waterscontaining the cobalt ions and the tungstic acid and/or its derivatives,those skilled in the art will be capable of selecting the most suitableconfiguration. Where not explicitly described otherwise, when referenceis made in this invention to a treatment performed in an ion-exchangecolumn this also means process configurations comprising two or moreion-exchange columns placed in series. Where the process according tothis invention is carried out in continuous mode, stage a) may also becarried out using two or more cation-exchange resins, preferably two ormore ion-exchange columns in parallel, which can work simultaneously oralternately, thus making it possible to regenerate the spent resinswithout interrupting the process.

As is known, cation-exchange resins and more generally ion-exchangeresins comprise a polymer matrix (generally granules of a fewmillimetres in diameter) in which ions available for ion exchange aretrapped or incorporated. In the process according to this inventioncrosslinked cation-exchange resins, more preferably of the strong acidtype, in which the acid functional group preferably comprises sulfonicgroups, are preferably used. Examples of cation-exchange resins of thestrong acid type used in the process according to this invention are theresins: Amberlite™ IR100 Na, Amberlite™ IR-118(H), Amberlite™IR-120(plus), Amberlite™ IR-120 Na, Amberlite™ IR-122Na, Amberlite™ 252Na, Amberlite™ SR1L Na Amberlyst™ XN-1010, Amberlyst™ 15WET, Amberlyst™36 WET, Amberjet™ 1200 Na, Amberjet™ 1000Na, Amberjet™ 1000(H),Amberjet™ 1300H, Amberjet™ 1300Na, Amberjet™ 4200 Cl, Amberjet™ 4600 Cl,Dowex® 50WX2-100, Dowex® 50WX2-200, Dowex® 50WX2-400, Dowex® 50WX4-50,Dowex® 50WX4-100, Dowex® 50WX4-200, Dowex® 50WX4-200R, Dowex® 50WX4-400,Dowex® 50WX8-100, Dowex® 50WX8-200, Dowex® 50WX8-400, Dowex® HCR-S,Dowex® HCR-W2, Dowex® 88, Dowex® 650C, Dowex Marathon™ C, DowexMarathon™ MSC-1, Duolite™ C-26.

Preferably the cation-exchange resins in stage a) of the processaccording to this invention are characterised by a concentration ofactive sites which is greater than or equal to 1.8 and less than orequal to 4.8, preferably greater than or equal to 4.4 and less than orequal to 4.7 meq/gram (determined on the anhydrous resin).

When stage a) of the process according to this invention is carried outin an ion-exchange column, the spent catalytic waters are fed to thesaid stage, preferably at a flow rate of between 1 and 50, morepreferably between 1 and 5, and even more preferably between 1 and 3BV*/h (LHSV). Preferably stage a) of the process according to thisinvention is carried out at a temperature of ambient temperature (25 C)or above and may be carried out up to the maximum temperature at whichthe cation-exchange resin can be used. In a preferred embodiment stagea) of the process according to this invention is carried out attemperatures of 15 to 85° C., preferably between 25 and 80° C. In apreferred embodiment when the tungstic acid and/or its derivatives is apolytungstate, stage a) is preferably carried out at temperatures of15-30° C., thus leading to particularly high yields of recovery ofcobalt ions and polytungstates.

As far as the pH of the spent catalytic waters fed to stage a) of theprocess according to this invention is concerned, this preferably liesbetween 2.5 and 4, more preferably between 2.7 and 3.0.

Stage a) of the process according to this invention is more effective interms of the adsorption of cobalt ions if, before they are placed incontact with the cation-exchange resin, the spent catalytic waters havebeen previously purified of any organic compounds which may be present.For example, when the catalytic waters which have to be fed to stage a)of the process according to this invention derive from processes of theoxidative cleavage of vegetable oils, these are separated from theprocess flow containing the organic phase with the reaction products ofthe said oxidative cleavage. Depending upon the quantity and the natureof any organic compounds which may be present, this preliminarypurification is carried out using techniques for the purpose which areknown to those skilled in the art, such as for example settling orliquid/liquid extraction.

In a preferred embodiment separation of the first organic phase fromstage a) may be carried out by settling. In order to assist settling, anorganic solvent, more preferably selected from the group comprisingn-hexane, n-heptane, n-octane, n-nonanoic acid and mixtures thereof,even more preferably n-octane, n-nonanoic acid and mixtures thereof arepreferably added. In a preferred embodiment the preliminary purificationto obtain the catalytic waters which have to be fed to stage a) of theprocess is carried out by settling in the presence of 5-20% by volume,preferably 7-12% by volume, of organic solvent with respect to the totalvolume of the flow which has to be purified. When carried out in thepresence of an organic solvent this settling is carried out attemperatures of ambient temperature (25° C.) or above, preferably atleast 5° C. below the boiling point of the organic solvent or anyazeotrope which the latter forms with water. When this settling iscarried out using n-octane as organic solvent the temperature preferablylies between 60 and 90° C.

Depending upon the starting characteristics of the spent catalyticwaters, one or more further pre-treatments selected from centrifuging,filtration, microfiltration, nanofiltration, ultrafiltration, osmosis orother suitable solid/liquid separation techniques and combinationsthereof may be carried out before stage a) of the process according tothis invention.

At the end of stage a) the cobalt ions originally present in the spentcatalytic waters are preferably almost completely adsorbed onto thecation-exchange resin. In fact, the catalytic waters separated out fromstage b) of this process have a cobalt ion content of less than 5 ppm,preferably less than 2 ppm. This cobalt ion concentration is in factsufficiently low not to have a prejudicial effect on subsequent stagesof the process and subsequent recovery of the tungstic acid and/or itsderivatives. In stage b) of the process according to this invention thecatalytic waters may be separated from the cation-exchange resinaccording to any method known to those skilled in the art for separatinga solid phase, such as ion-exchange resin, from a liquid phase, forexample by means of filtration, centrifuging, sedimentation, or usingany combination of these methods. This separation may be performed inequipment other than that in which stage a) has been carried out, or inthe same equipment. For example, when stage a) of the process is carriedout in an ion-exchange column, separation of the catalytic waters fromthe cation-exchange resin typically takes place in the terminal part ofthe column, for example through a septum which retains the resin andallows the catalytic waters to flow out.

Depending upon the concentration of tungstic acid and/or its derivativesin the catalytic waters separated from the cationic resin, one or moretreatments for concentrating the said catalytic waters in order toremove part of the water present are carried out in stage c) of theprocess according to this invention. This stage c) may be carried outusing any method known to those skilled in the art, for example by oneor more water evaporation treatments. Those skilled in the art arecapable of identifying the desirable operating conditions, for examplepressure and temperature, to remove the appropriate quantity of water.

Typically the aqueous solution obtained at the end of stage c) of thisprocess has a concentration of tungstic acid and/or its derivatives,expressed as tungsten concentration, of between 10 and 15% by weight anda concentration of cobalt ions of less than 50 ppm. Surprisingly, it hasbeen discovered that such an aqueous solution can be used forpreparation of the catalytic solution for the hydroxylation stage inoxidative cleavage processes such as those described in WO 2008/138892and WO 2011/80296, making it possible to obtain reaction yields, underthe same reaction conditions, which are wholly comparable with thosewhich can be achieved using fresh catalytic solutions of tungstic acidand/or its derivatives. This makes it possible to use the processaccording to this invention as a treatment for the recovery andregeneration of catalysts in such oxidative cleavage processes, thushelping to reduce the costs of the disposal of catalytic waters and theenvironmental impact of these processes.

The concentrated aqueous solution containing tungstic acid and/or itsderivatives obtained at the end of stage c) of this process, may befurther treated to recover tungstic acid and/or its derivatives. Such arecovery stage is advantageously carried out using one or moreseparation treatments, for example by distillation, liquid/liquidextraction, adsorption, precipitation, crystallisation or combinationsthereof. Those skilled in the art are capable of selecting theappropriate method of recovery according to the concentration of thetungstic acid and/or derivatives present in the starting aqueoussolution.

Preferably, the recovery stage of the process according to thisinvention is carried out by means of a treatment precipitating out thetungstic acid and/or its derivatives.

Precipitation of tungstic acid and/or its derivatives may be achieved,for example by progressively concentrating the acids by evaporating thewater or reducing solubility, for example by lowering the pH ortemperature of the aqueous solution. It is also possible to combineseveral methods of precipitation, for example by first concentrating theacids through the evaporation of water and subsequently reducing theirsolubility by reducing the temperature of the remaining aqueoussolution.

The precipitate obtained can then be separated from the aqueous solutionby means of any of the methods known to those skilled in the art, forexample by filtering or centrifuging, or using any combination of thesemethods.

The tungstic acid and/or its derivatives separated out at the end of therecovery stage can subsequently be further purified. The saidpurification stage may be performed by means of one or more treatmentsselected from drying, lyophilisation, distillation, liquid/liquidextraction, adsorption or crystallisation.

In stage d) of the process according to this invention thecation-exchange resin separated from the catalytic waters at the end ofthe stage b) is placed in contact with an acid aqueous solution. Thisbrings about elution of the adsorbed cobalt ion and at the same timeregenerates the cation-exchange resin which can be reused in subsequenttreatments according to stage a) of this process. Preferably the saidacid aqueous solution is prepared from an acid preferably selected fromthe group comprising hydrochloric acid, sulfuric acid, phosphoric acid,hydrobromic acid, acetic acid, or even more preferably sulfuric acid.

Said stage d) may be performed in an item of equipment which isdifferent from that in which stage a) is performed, or in the sameequipment. For example when stage a) of the process is carried out in anion-exchange column, elution of the cobalt ions from the cation-exchangeresin is typically carried out in the same column, feeding acid aqueoussolution to it.

Those skilled in the art will be capable of identifying suitablequantities of acid aqueous solution, its pH and contact time, in orderto quantitatively elute cobalt ions from the cation-exchange resin. Acidaqueous solution characterised by a pH of less than 0.5 may preferablybe used. In a preferred embodiment of this invention elution of cobaltions from the cation-exchange resin is carried out by using up to 10parts by weight per part of cation-exchange resin of an acid aqueoussolution having a pH of less than 0.5.

In order to maximise the elution yield stage d) of the process accordingto this invention may be repeated more than once, each time feeding oneor more aliquots of fresh acid aqueous solution, preferably with aprogressively decreasing pH.

When stage a) is carried out in an ion-exchange column, the elution ofcobalt ions may be further favoured by feeding the acid aqueous solutionin countercurrent flow with respect to the flow of catalytic waters fedduring stage a). This makes it possible to minimise the volume of acidaqueous solution required to elute the cobalt ions.

The aqueous solution obtained at the end of stage d) of this process maybe used as such, or may be concentrated or diluted and may be furtherpurified to remove the excess acid in order that it may subsequently beused to prepare the catalytic solution in the oxidation stage ofcatalytic cleavage processes, such as those described in WO 2008/138892and WO 2011/80296, obtaining reaction yields which for the same reactionconditions are wholly comparable with those which can be obtained usingfresh catalytic solutions containing cobalt.

The aqueous solution obtained at the end of stage d) of this process mayalso be further treated to recover the cobalt present therein. Such arecovery stage may advantageously be carried out using one or moreseparation treatments, for example by distillation, complexing,adsorption, precipitation, crystallisation, electrolysis or combinationsthereof. Those skilled in the art will be in a position to select theappropriate means for recovery, depending upon the concentration ofcobalt ions present in solution and the form in which they wish torecover the cobalt.

In a preferred embodiment of the process according to this invention,the recovery stage of the process according to this invention is carriedout by means of treatment precipitating out the cobalt in the form ofhydroxide which is subsequently filtered and redissolved in the presenceof an acid, preferably selected from the group comprising hydrochloricacid, sulfuric acid, hydrobromic acid, phosphoric acid or acetic acid,more preferably acetic acid, thus reobtaining a cobalt salt such as, forexample cobalt acetate, cobalt chloride, cobalt sulfate, cobaltphosphate or cobalt bromide. In a preferred embodiment the cobaltrecovery stage is carried out by precipitating out the cobalt ashydroxide, filtering the said hydroxide and redissolving it in aceticacid to obtain cobalt acetate.

It is also possible to achieve precipitation of the cobalt by adding asalt comprising a counterion capable of forming an insoluble salt withcobalt (precipitation by ion exchange) to the aqueous solution. It isalso possible to combine several methods of precipitation, for examplefirst concentrating the aqueous solution by evaporating the water andsubsequently reducing the solubility of the salt by lowering thetemperature of the remaining aqueous solution.

The precipitate obtained can then be separated from the aqueous solutionby means of any of the methods known to those skilled in the art, forexample by filtering or centrifuging, or using any combination of thesemethods.

The cobalt salt separated out at the end of the recovery stage may thenbe subsequently purified. The said purification stage may be carried outusing one or more treatments selected from drying, lyophilisation,distillation, liquid/liquid extraction, or adsorption crystallisation.The invention will now be described through an Example which is intendedto be for illustrative purposes and does not limit the invention.

Example 1

28 kg of process flow originating from stage b) of a process for theoxidative cleavage of sunflower oil having a high oleic acid contentcarried out in accordance with Example 1 in application WO 2008/138892were reheated to 80° C. in the presence of 2.8 kg of n-octane andallowed to settle until an organic phase had separated out, thusobtaining 8.4 litres of spent catalytic waters containing approximately2400 ppm of cobalt ions and tungstic acid and/or its derivatives in aconcentration corresponding to approximately 8000 ppm of tungsten.

Subsequently the spent catalytic waters obtained after separation of theorganic phase were fed by gravity at 25° C. and BV/H = 2 to anion-exchange column (height 1 m and diameter 40 mm) packed with 216.2grams (dry weight, corresponding to 800 ml of wet resin) of Amberlyst™15WET resin (concentration of active sites on the dry resin = 4.7 eq/kg;concentration of active sites on the wet resin = 1.8 meq/ml; surfacearea 53 m²/g, pore diameter = 300 Å).

During the treatment it was possible to establish that cobalt waseffectively being adsorbed by the resin by observing progressivecolouration of the resin bed. Table 1 shows details of the amount ofadsorption (expressed as cm of colouration of the resin bed) as afunction of the volume of the catalytic waters fed and the residualcobalt concentration in the aqueous solution obtained at the exit fromthe column, once separated from the cation-exchange resin.

TABLE 1 Residual cobalt concentration in waters separated from the resin(ppm) Cobalt adsorption (cm colouration of the resin bed) Catalyticwaters fed (ml) 0.15 25 1766 0.08 41 3561 0.06 60 5242 0.03 79 7256 0.1686 7806 0.11 92 8216 0.06 93 8423

The aqueous solution separated from the cation-exchange resin wassubsequently concentrated by evaporating off approximately 90% of thewater, thus obtaining an aqueous solution containing tungstic acidand/or its derivatives in a concentration corresponding to approximately80000 ppm of tungsten. The said solution was used as a catalyticsolution in a reaction for the hydroxylation of vegetable oils, andrevealed no differences of any kind with respect to a fresh solutionhaving the same concentration.

6.3 kilograms of an aqueous solution of sulfuric acid (sulfuric acidconcentration 6.5% by weight) were fed countercurrently to theion-exchange column containing the cation-exchange resin on which thecobalt was adsorbed, separating out an aqueous solution containingapproximately 3150 ppm of cobalt ions, indicating almost total recoveryof the cobalt initially present in the spent catalytic waters.

Example 2

Example 1 was repeated, feeding 8.6 litres of the same spent catalyticwaters containing 2400 ppm of cobalt ions and tungstic acid and/or itsderivatives in a concentration corresponding to approximately 8000 ppmof tungsten to the ion-exchange column containing the cation-exchangeresin from which the cobalt had been eluted at the end of Example 1.

During the treatment it was possible to verify that cobalt waseffectively being newly adsorbed from the resin through observingprogressive colouration of the resin bed. In a similar way to Example 1,Table 2 shows details of the amount of adsorption (expressed as cm ofcolouration of the resin bed) as a function of the volume of thecatalytic waters fed and the residual cobalt concentration in theaqueous solution obtained at the exit from the column, once separatedfrom the cation-exchange resin.

TABLE 2 Residual cobalt concentration in waters separated from the resin(ppm) Cobalt adsorption (cm colouration of the resin bed) Catalyticwaters fed (ml) 0.32 13.5 863 0.55 32 2702 0.61 54 4854 0.72 76.5 67500.69 92 8667

The aqueous solution separated out from the cation-exchange resin wasthen subsequently concentrated by evaporating approximately 90% of thewater, thus obtaining an aqueous solution containing tungstic acidand/or its derivatives in a concentration corresponding to approximately80000 ppm of tungsten. The said solution was used as a catalyticsolution in a reaction for the hydroxylation of vegetable oils withoutdifferences of any kind in comparison with a fresh solution at the sameconcentration.

2.1 kilograms of an aqueous solution of sulfuric acid (sulfuric acidconcentration 6.5% by weight) were fed countercurrently to theion-exchange column containing the cation-exchange resin on which thecobalt was adsorbed, separating out an aqueous solution containing 9830ppm of cobalt ions, indicating almost total recovery of the cobaltinitially present in the spent catalytic waters and maintaining theeffectiveness of the cationic resin.

1. A process for oxidative cleavage of vegetable oils containing monounsaturated fatty acid triglycerides comprising: 1) a first step of hydroxylation of the double bond present in the monounsaturated fatty acid triglycerides in the presence of a catalyst comprising tungstic acid and/or its derivatives, to yield a vicinal diol, 2) a subsequent second step of oxidation of the vicinal diol in the presence of catalysts comprising cobalt salts, thereby obtaining a mixture comprising saturated monocarboxylic acids and saturated carboxylic acid triglycerides having more than one acid group, 3) a step of separation of an aqueous solution containing cobalt ions and tungstic acid and/or their derivatives from the mixture of step 2) and 4) a step of separation of cobalt ions from said aqueous solution of step 3) comprising the operations of: a) removing the cobalt ions by placing the aqueous solution in contact with a cation-exchange resin; b) separating the aqueous solution from the cation-exchange resin; c) concentrating the aqueous solution obtained from stage b, thereby obtaining a concentrated aqueous solution containing tungstic acid and/or its derivatives, d) eluting cobalt ions from the cation-exchange resin in stage b, using an acid aqueous solution, thereby obtaining an aqueous solution comprising cobalt ions, wherein the aqueous solution obtained at the end of step 4-d) is used to prepare the catalyst for the oxidation step 2).
 2. The process according to claim 1 wherein the tungstic acid derivatives of step 1) are selected from the group consisting of phosphotungstic acid, pertungstic acid and polytungstates and their cobalt salts.
 3. The process according to claim 1 wherein the cobalt salts of step 2) are selected from the group consisting of cobalt acetate, cobalt chloride, cobalt sulfate, cobalt nitrate and cobalt bromide.
 4. The process according to claim 1, wherein at the end of step 4-c) the concentration of tungstic acid and/or its derivatives in the concentrated aqueous solution, expressed as tungsten concentration, is of between 10 and 15% by weight and the concentration of cobalt ions is of less than 50 ppm.
 5. The process according to claim 1, comprising before step 4-a) a preliminary purification step for separating an organic phase from the aqueous solution.
 6. The process according to claim 5, in which the separation of the organic phase is performed by decantation.
 7. The process according to claim 6, in which the decantation is performed in presence of an organic solvent.
 8. The process according to claim 7, in which the organic solvent is selected from the group consisting of n-hexane, n-heptane, n-octane, n-nonanoic acid and mixtures thereof.
 9. The process according to claim 8, in which the organic solvent is n-octane, nonanoic acid or mixtures thereof.
 10. The process according to claim 1, in which the cationic exchange resin is crosslinked.
 11. The process according to claim 1, in which the cation-exchange resin is of the strong acid type.
 12. The process according to claim 11, in which the functional group of the cation-exchange resin of the strong acid type is constituted by sulphonic groups.
 13. The process according to claim 1, in which the step 4-a) is performed with an ionic exchange column.
 14. The process according to claim 13, in which the aqueous solution is fed to step a) with a flow rate comprised between 1 and 50 BV*/h (LHSV).
 15. The process according to claim 2, in which the step 4-a) is performed with an ionic exchange column.
 16. The process according to claim 3, in which the step 4-a) is performed with an ionic exchange column.
 17. The process according to claim 4, in which the step 4-a) is performed with an ionic exchange column.
 18. The process according to claim 5, in which the step 4-a) is performed with an ionic exchange column.
 19. The process according to claim 6, in which the step 4-a) is performed with an ionic exchange column.
 20. The process according to claim 7, in which the step 4-a) is performed with an ionic exchange column. 